The present invention relates to single phase brushless DC motors, i.e. a motor with a single coil. Single-phase motors are typically used in low cost motor applications, such as fan cooling applications, which come in two flavors: (a) without speed control, and (b) with speed control. In case (a) the fan is simply switched ON or OFF. In case (b) there is typically a remote CPU involved which provides a signal indicative of the desired speed, typically in the form of a PWM signal, while the motor driver itself basically converts the input PWM signal into an output PWM signal according to some look-up table or transfer function.
For many applications, three phase brushless DC motors are preferred because of their lower torque ripple, leading to lower noise, higher efficiencies and higher start up torque. But single coil motors are cheaper to produce and to drive, and therefore preferred in some high-volume markets, such as e.g. fans for cooling CPU's in desktops, refrigerators, printers, or fans in automotive applications, as a few examples only.
Brushless DC motors have the advantage that no brushes are needed, but they require a specific driving scheme, called “electrical commutation” to change the direction of the current through the one or more coils, which principle is well known in the art.
A specific market requirement of speed control fans is the definition of the speed curve. This speed curve defines how an input signal, for instance a duty cycle input signal (DCin) is converted into a resulting fan motor speed. In case of fan-drivers which control power stages using PWM to control the coil current, the coil energization is the result of a waveform generated. As is known in the art, PWM control offers many advantages, inter alia: low CPU load, possibility to generate or approximate virtually any waveform (e.g. sinusoidal waveform), it allows that transistors of the drive stage (e.g. containing dual H-bridges) are driven either in their ON state or in their OFF state (but not in their “linear region”) thereby reducing heat dissipation. Preferably a relatively high switching frequency is used, for example at least 16 kHz or at least 20 kHz to avoid acoustical noise caused by switching currents. The higher the PWM frequency, the better a target waveform can be approximated, typically resulting in lower torque ripple.
A well known problem caused by the switching of motor currents having a magnitude in the order of a few hundred mAmp or more, is that these currents may cause EMC related issues (due to conducted emissions and radiated emissions), which problems are typically addressed using addition of appropriate filters, such as for example pi-filters, containing at least one coil and at least two capacitors.
Another way of driving a motor at a variable speed is by driving the power transistors of the drive stage in their linear region, e.g. by slowly turning them “ON” and “OFF”. This also offers the advantage of approximating a target waveform very accurately, but has the disadvantage that it dissipates more power in the transistors, which reduces the power efficiency, and requires active or passive cooling.
The present invention is related to fans in applications which are sensitive to electromagnetic coupled (EMC) noise, for instance in an automotive environment, which is a high-volume and highly competitive market, where cost is sometimes more important than secondary aspects, such as acoustical noise.
FIG. 2 shows a typical system configuration where a remote processor 21 provides a duty cycle signal (DCin) as a PWM input-signal to the single coil motor driver. Typically the remote processor modifies this PWM input signal based on the speed feedback it receives from the single coil motor driver, typically in the form of a Frequency signal (referred to as FG signal). The single coil motor driver should provide a monotonous relationship between the PWM input and the speed of the fan. This relationship can be a rising speed for a rising PWM input duty cycle, or the relationship can be inverse, where a rising speed implies a reducing speed. Even though such monotonous relationship is not absolutely mandatory, it allows a remote processor to easily close the speed control loop.
FIG. 3 shows a characteristic of such a prior art low cost fan-driver, designed for providing a PWM signal with an output duty cycle (DCout) signal in order to modify the applied drive current. The speed resulting from the energization depends on the fan design, such as the blade design, and system environment, such as back pressure, applied supply voltage, and the present ambient temperature. The lack of closed loop speed control, and the non-linear increase of the load as a function of the speed, causes that the speed curve is a non-linear function of the output duty cycle (DCout), but several applications accept a natural speed curve in which the percentage value of duty-cycle-in (DCin) is equal to that of DCout. This relationship is quite easy to realize in a state-machine and results in very low-cost speed controlled fan-drivers.
In systems like FIG. 2 the remote processor can regulate the fan speed such as to compensate for environmental change, or fan tolerances, based on the FG signal feedback, and the natural speed curve of the low cost fan driver.